1. Field of the Invention
The present invention relates to spark plugs for internal combustion engines. More particularly, the present invention relates to a spark plug having a protective coating on a metal shell portion thereof, and to a method of making such a spark plug.
2. Description of the Background Art
Spark plugs are widely used to ignite fuel in internal combustion engines. Spark plugs of many types are known and are commercially available. Spark plug electrodes are subject to intense heat and to a highly corrosive environment generated by the exploding air/fuel mixture. To improve durability and erosion resistance, spark plug electrode tips must be able to withstand the high temperature and corrosive environment resulting from the chemical reaction products between air, fuel, and fuel additives within a combustion chamber.
Spark plugs generally include a hollow ferrous metal shell, and a ceramic insulator partially enclosed within the shell. This spark plug shell usually has male threads formed on the outside thereof The metal shell is the portion of the spark plug which engages with a threaded hole in an engine cylinder head when the spark plug is rotatably installed therein.
A problem of xe2x80x98seizingxe2x80x99 sometimes occurs, in which the metal shell portion of a spark plug, normally made of iron or steel, may become locked in place in an aluminum cylinder head, over time, if left undisturbed therein. Since cylinder heads are commonly made out of aluminum-based alloys in most modern internal combustion engines, this potential for seizing is of concern.
Seizing of this type is particularly a risk where spark plugs are left in place for extended time intervals. Automotive manufacturers are now selling vehicles with engines that can go many thousands of miles between tune ups, and as a result, spark plugs are being left in useful service for extended time periods. Durability and corrosion resistance are also important concerns in such extended life spark plugs.
Accordingly, a need exists for an improved spark plug, which is resistant to seizing in place in internal combustion engines, particularly in aluminum alloy cylinder heads. Preferably, such a spark plug would be seize-resistant even where left in place for extended service intervals. Most preferably, such a seize-resistant spark plug would also have superior durability and corrosion resistance.
Titanium compounds have been suggested for use as one component of an internal spark plug resistor by Yamada et al. in U.S. Pat. No. 4,173,582, and have been used as coatings to harden some tools such as cutting tools, but are not presently used as coatings for spark plugs.
The present invention provides a method of making a spark plug having a metal shell with a thin protective coating thereon, in which the protective coating is resistant to the spark plug""s seizing in place. The preferred coating includes a titanium compound. The present invention also provides a spark plug which is a product of the described method.
Optionally, the protective coating hereof may be a two-part coating comprising an inner coating layer, which comprises a corrosion-resistant material, and an outer coating layer which comprises a titanium compound. Where the two-part coating is used, a preferred material for the inner coating layer is selected from the group consisting of zinc, zinc chromate, nickel, and nickel alloys.
The method of coating a metal spark plug shell with a titanium-containing compound involves placing the spark plug shell into a vacuum chamber, lowering the pressure in the chamber to a level below atmospheric pressure, and applying a protective coating, containing a titanium compound, to a portion of the exterior of the spark plug shell by physical vapor deposition.
The coating on the spark plug shell improves corrosion resistance thereof, provides a pleasing appearance, and resists seizing of the spark plug in place in a cylinder head portion of an engine.
Accordingly, it is an object of the present invention to provide an extended life spark plug having a seize-resistant coating on the threaded base thereof,
It is another object of the invention to provide an extended life spark plug of the type described which also exhibits improved durability and corrosion resistance as compared to known spark plugs.
It is a further object of the present invention to provide a method of making a seize-resistant spark plug.
For a more complete understanding of the present invention, including further objects, features, and advantages, the reader is referred to the following detailed description section, which should be read in conjunction with the accompanying drawings. Throughout the following detailed description and in the drawings, like numbers refer to like parts.